Final drive and Differential part 1

Final Drive

Final drive is the last section of the gear train and forms part of the drive line of the motor vehicle     (Moodle). It often consist of a pinion gear driving a big ring gear( or crown wheel) and the final drive ratio is calculated by ring gear teeth divide by pinion gear teeth. Its main purpose is to slit up drive into 2 paths to each wheel drive. This mechanism is done by a pinion gear set longitudinally to the engine layout but perpendicularly to the crown wheel creating a 90 deg drive transmit.
Final drive literally mean final gear drive ratio works just like gears drive in the gear box, for example: normally pinion/crown 3.8:1; 4.11:1 will give you higher engine revs, faster acceleration, more pulling power, less gas mileage and slower top speed. 3.73:1 will give you slightly lower engine revs, slower acceleration, less stump pulling power, slightly better gas mileage and a higher theoretical top speed.
Final drive "gears down" the engine speed, allowing the direct gear drive (4th) to be available in the gear box( very important).

From here, I've learned that there are different types of final drive set up:
    a. Worm drive: basically is a screw with transverse thread along its body mesh with the crown wheel teeth. By this cause the pinion can only be placed somewhere above or below the crown wheel. I can see its only advantage is to prevent reward movement caused when car is @ slope. Otherwise this is not very ideal because it doesn't have deep mechanical contact, n slow as well therefore no good for heavy duty vehicle nor performance cars. Fortunately this is only the design of the past to modern vehicles.
    b. Spiral Bevel gear: Helical teeth, pinion placed at middle height of the crown wheel,  solving the worm's problem of drive shaft's height. Also, helical teeth allows it to operate smoother and more quiet, also better contact and drive efficiency.
From the spiral bevel, HYpoid gear set is the next generation, with helical teeth both crown n pinion but a lower set up from the centre line of the crown wheel. The number of contacting teeth is actually higher, allowing better contact, meshing leading to a higher strengths load and a higher gear reduction ratio. => Overall better drive efficiency. This is why hypoid gear final drive is most common application for  modern vehicle.
"For practical purposes, it is often impossible to replace low efficiency hypoid gears with more efficient spiral bevel gears in automotive use because the spiral bevel gear would need a much larger diameter to transmit the same torque. Increasing the size of the drive axle gear would require an increase of the size of the gear housing and a reduction in the ground clearance"( Source: wikipedia: http://en.wikipedia.org/wiki/Spiral_bevel_gear#Comparison_of_spiral_bevel_gears_to_hypoid_gears). This is the reason why hypoid gear possesses more strength than the spiral because it can deliver more torque with the number of contacting teeth it has.
And that's all I seem to be able to put on about final drive right now.